1. Field of the Invention
The present invention relates generally to semiconductor devices and a method of fabricating the same. More particularly, the present invention relates to a method of fabricating a recessed channel access transistor (RCAT) device with reduced GIDL (gate induced drain leakage). The invention is particularly suited for high-density dynamic random access memory (DRAM) applications.
2. Description of the Prior Art
As the size of semiconductor devices shrinks, the gate channel length decreases correspondingly. Consequently, a short channel effect may occur. The conventional method of solving the short channel effect includes decreasing the thickness of the gate oxide layer or increasing concentration of the dopants. These methods, however, may deteriorate the reliability of the devices and decrease the speed of transferring data.
In the last few years, recessed channel access transistor devices (or RCAT devices in short) have been developed to suppressing the short channel effect by physically increasing the gate channel length without an increase in a lateral area of a gate electrode. The RCAT transistor has a gate oxide layer formed on sidewalls and the bottom surface of a recess etched into a substrate, where a conductive substance fills the recess, contrary to a planar gate type transistor having a gate electrode formed on a planar surface of a substrate. Therefore, the integration of the recessed-gate transistor can be increased.
However, in a conventional RCAT device, when a drain voltage (Vd) is applied to a capacitor that is electrically connected to an NMOS transistor, a gate induced drain leakage (GIDL) problem may occur. A sharp upper corner of the poly gate and the relative thinner gate oxide layer near the sharp corner of the poly gate result in a concentrated electric field at the cell side of a DRAM cell. The concentrated electric field leads to the leakage. The GIDL adversely affects the refresh characteristic of the DRAM device. Therefore, it is desirable to remove or alleviate the GIDL in the RCAT device in order to improve the refresh characteristic of the DRAM device.